This invention relates generally to broadband communications systems, such as cable television systems, and more specifically to burst-mode combining of reverse path radio frequency (RF) signals that are generated in the broadband communications systems.
FIG. 1 is a block diagram illustrating an example of a conventional broadband communications system 100, such as a two-way hybrid fiber/coaxial (HFC) communications system, that carries optical and electrical signals. Such a system may be used in a variety of networks, including, for example, a cable television network; a voice delivery network; and a data delivery network to name but a few. The communications system 100 includes a headend facility 105 for generating forward, or downstream, radio frequency (RF) signals (e.g., video, voice, or data signals) that are transmitted in a forward frequency band. A typical forward frequency band ranges from 50 Mega Hertz (MHz) to 860 MHz. Numerous application devices 110, 175, 176, 177, 178, 179 located within the headend facility 105 generate the forward RF signals. For example, a digital network control system (DNCS) 110 controls the routing of digital video broadcast signals and provides the signals to, for example, quadrature amplitude modulation (QAM) modulators 115a-n and/or digital audio/visual council (DAVIC) modulators 120 that modulate the signals with a desired forward carrier signal. A combiner 125 combines the modulated RF signals with other modulated signals being supplied from other modulators and provides the signals to a broadcast optical transmitter 130. In a known conventional manner, the broadcast optical transmitter 130 first converts the signals to an optical signal and an erbium-doped fiber amplifier (EDFA) 135 then amplifies the optical signal. A splitter 140 then splits the optical signal for transmission downstream through a long haul fiber distribution network 145.
A forward optical receiver (FORU) (not shown) that is included in each of a plurality of fiber nodes 150a-h receives the split optical signal and converts the signal back to RF signals in a known manner. The RF signals are then routed through an RF distribution network 155 for delivery to connected network terminal devices 160a-h. It will be appreciated that the network terminal devices 160a-h can be a variety of different communication devices that are tuned to receive the broadcast RF signals at specific forward frequencies. By way of example, device 161 may be a cable modem tuned to receive signals that include DOCSIS cable modem termination system (CMTS) signals; device 162 may also be a cable modem tuned to receive signals that include pre-DOCSIS CMTS signals; device 163 may be a status monitoring device that receives status monitoring signals; and device 164 may be a telephone that receives cable telephone signals, to name but a few.
In the reverse frequency band, which typically ranges from 5 MHz to 42 MHz, electrical signals are provided from the network terminal devices 160a-h to the headend facility 105 through the RF and fiber distribution networks 155, 145. Periodically, the network terminal devices 160a-h each sends reverse carrier signals in predetermined reverse frequency bands to the application devices. It will be appreciated, however, that these reverse carrier signals are not sent by the network terminal devices 160a-h at all times. This periodic transmission of carrier signals is colloquially known in the art as xe2x80x9cburst modexe2x80x9d transmissions. Moreover, the normal functioning and protocol of each application device 110, 175-179 controls the timing of the reverse carrier signals. For example, the DNCS 110 allows one set-top device to transmit signals at a specific frequency at a specific time and, when provided, receives the reverse carrier signal from the set-top device via DAVIC modulator 180. This conventional reverse protocol insures that there is no ambiguity by the application devices 110, 175-179 as it receives signals from the plurality of network terminal devices 160a-h. FIG. 2 illustrates a typical reverse band and the frequencies allocated to various services that may be used by the network terminal devices 160a-h for the purpose of sending reverse carrier signals.
Unfortunately, however, in addition to the desired reverse carrier signals that are sent through the networks 155, 145, unwanted noise signals also enter the RF distribution network 155 by numerous means and conditions. A large portion of the unwanted noise signals enter the system through, for example, defective connectors, poorly shielded cable, and other cable components located at the subscriber location or throughout the RF distribution of the network 155. Consequently, these unwanted noise signals degrade the ability of the respective application device 110, 175-179 to effectively process the desired reverse carrier signals.
A reverse optical transmitter (ROTU) (not shown) is also included in each of the plurality of fiber nodes 150a-h. The ROTU converts the reverse RF signal(s), which includes both the carrier signals and the noise signals, to an optical signal and provides the optical signal via the fiber distribution network 145 to a corresponding reverse optical receiver (RORU)165a-h. It will be appreciated that separate reverse fiber paths (not shown) are routed between each of the reverse optical transmitters (ROTUs) and the respective reverse optical receiver (RORU) 165a-h. Typically, this is required because reverse optical signals of the same wavelength cannot conventionally be combined and, therefore, require a direct fiber link between an optical transmitter to an optical receiver in the reverse path.
The RORUs 165a-h each convert the optical signals back to electrical signals in a conventional manner. The reverse signals provided by each of the RORUs 165a-h are then electrically combined through passive combiner 170. Application devices 110, 175-179 are tuned to a specific reverse frequency band (e.g., 205, 210, 215, 220 (FIG. 2)) in order to receive just the desired portion of the combined reverse signals, which includes the desired carrier signal(s). By way of example, a DOCSIS CMTS 175 may be tuned to receive carrier signals within reverse frequency band 205, a status monitoring device 177 may be tuned to receive carrier signals within reverse frequency band 210, a cable telephone device 178 may be tuned to receive carrier signals within reverse frequency band 215, and a pay-per-view device 179 may be tuned to receive carrier signals within frequency band 220. Commonly eight to ten independent application devices offering specific services utilize the return frequency band. Each of these applications orchestrates the timing of their associated network terminal device (e.g., 160a-h) such that only one network terminal device transmits within the application""s return frequency band at a time. This orchestration of singular transmission within a reverse frequency band may also be used to orchestrate the behavior of elements that are or are not the linking application to its targeted network terminal device.
Unfortunately, as mentioned, noise signals, also referred to as ingress signals, can enter the system at any time and travel to the headend facility 105, regardless of whether or not a desired reverse carrier signal is being transmitted. Once ingress signals are present in the system, the ingress signals are transmitted back through the HFC reverse path along with any desired carrier signal(s). Of particular concern is the fact that the undesired ingress signals from multiple premises tend to be combined through the system and, therefore, to build in relative amplitude. The aggregate of these undesired ingress signals could pose a considerable threat to the ability of the system to successfully transmit and process the desired carrier signals. More specifically, after conversion back to electrical signals, the ingress signals and the desired carrier signals are combined with other reverse signals transmitted by the RORUs 165a-h via the passive combiner 170. As a result, ingress signals delivered from each of the RORUs 165a-h that have been combined into one reverse signal reduces the desired carrier to noise signal strength ratio (CNR). A low CNR can effectively render the desired carrier signals useless or force the operator to use signal encoding methods that are slower, but may be more immune to the effects of noise. Additionally, it is known that the RORUs 165a-h each output signals in the entire reverse signal band at all times to facilitate the instantaneous receipt of a carrier signal from any one of the connected network terminal devices 160a-h. Consequently, ingress signals that have entered the system are being continuously transmitted from each of the RORUs 165a-h to the combiner 170 at all times.
What is needed, therefore, is a device that selectively blocks the output signals of each individual RORU 165a-h prior to combining the output signals, unless the RORU 165a-h is outputting a desired reverse carrier signal. In this manner, performance that is adversely affected by the aggregate ingress signals can be improved by mitigating a portion or all of the unwanted ingress signals. More specifically, such a device would limit ingress signals reaching the target application device to that of just the RORUs 165a-h that are transmitting a carrier signal(s) rather than the additive condition previously described. By blocking the unwanted ingress noise from the RORUs 165a-h that are not transmitting a reverse carrier signal, the CNR related to transmitted reverse carrier signals is significantly improved resulting in a more robust or enhanced operation of the targeted application device.